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superdavelotus

Header Tank Cap Pressure?

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Hi all, my V8 has been running slightly warm these recent weeks and someone told me to start by replacing the header tank pressure cap. Iam not that technical but thought for a few quid it's worth replacing on a regular basis anyway.

I've just checked my old cap and it's a 145KPa rated. Just had quick look in manual and it should be a 110KPa. Iam not sure what this means? Does a higher rated pressure cap mean it will run higher or raise the boiling point? Why would anyone fit a higher rated cap on, is it a good or bad thing?

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Dave,

No idea really but presume it means your existing cap would have witheld an additional 35 Kpa of pressure before blowing and releasing that pressure. Again, I'm guessing, but an extra 35Kpa before blow-out doesn't sound good, hence Lotus decision to get the cap to blow at 110Kpa.

'Course, I could be talking bollocks so I'll leave this to the technical lads.

Graham.


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North Midlands Esprit Group<br /><br />
NMEG "the formidable squadron"<br /><br />
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A higher pressure means a higher boiling point. I think with a higher pressure cap the engine would get much hotter before the coolant boiled and the cap released the pressure. I can't see this being a good thing - running the engine hotter doesn't seem like a good idea to me :P

You should also check the expansion tank cap (under the offside wheel arch) that may have the lower pressure cap on so it would release pressure before the other. This might make sense since then you would be releasing coolant over the wheel rather than from the header tank all over the engine.


Dave - 2000 Sport 350

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The expansion tank doesn't have a pressure cap only a blanking cap.

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Hi Dave,

My understanding is that yes the water would boil at a higher temperature, but that this will not afect the normal operating temperature of the engine.

I suspect that the operating efficiency of the radiator its-self is being reduced by debris in the fins.

My first check would be to drop the radiator and clean the fins.

I did see on an american members site that it can be done with the car on axle stands and pivoting the radiators apart (air con and water rads). Cant remember site - sorry

Don't forget the engine temp may run hotter when the air con is being used.

Cheers

Glyn

Edited by glynherron

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Hi Dave,

My understanding is that yes the water would boil at a higher temperature, but that this will not afect the normal operating temperature of the engine.

I suspect that the operating efficiency of the radiator its-self is being reduced by debris in the fins.

My first check would be to drop the radiator and clean the fins.

I did see on an american members site that it can be done with the car on axle stands and pivoting the radiators apart (air con and water rads). Cant remember site - sorry

Don't forget the engine temp may run hotter when the air con is being used.

Cheers

Glyn

A higher pressure cap will raise the boiling point and thus keep the car running cooler. When the water boils it creates air pockets which prevent the water/coolant from performing it's supposed function of pulling heat from the engine. If there are no air pockets in the water(ie not boiling) then the water can pull the heat away from the engine and thus runs cooler. I currently changed my stock 7psi pressure cap and raised it to 13psi(don't know the conversion of kilopascals(kpa) to psi sorry) and the car runs about 15degrees F cooler than previously. You could also drain some coolant and dilute your mixture to about 65% water and 35% coolant if it's a standard 50/50 mix. OR you could drain the radiator entirely and add an additive I've used for years in my Road Race RX7 called Pro Blend "cools like ice water" if they sell it over there. Good luck.

Artie


89 White Esprit SE

...a few little upgrades....

93 RX7.....Silverstone

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A higher pressure cap will raise the boiling point and thus keep the car running cooler. When the water boils it creates air pockets which prevent the water/coolant from performing it's supposed function of pulling heat from the engine. If there are no air pockets in the water(ie not boiling) then the water can pull the heat away from the engine and thus runs cooler. I currently changed my stock 7psi pressure cap and raised it to 13psi(don't know the conversion of kilopascals(kpa) to psi sorry) and the car runs about 15degrees F cooler than previously. You could also drain some coolant and dilute your mixture to about 65% water and 35% coolant if it's a standard 50/50 mix. OR you could drain the radiator entirely and add an additive I've used for years in my Road Race RX7 called Pro Blend "cools like ice water" if they sell it over there. Good luck.

Artie

Hi Artie

I agree that the water would boil at a higher temp and the damaging pockets of air caused during goiling would occur at a higher temperature.

If the thermostat opens at say 82 degrees, surely 82 degrees is 82 degrees? So how will the engine run cooler?

I would have thought that a higher pressure cap would allow for higher temps to be reached before the 'overheating' damage takes place, ie the gauge could show hotter before damage occurrs.

If I am misunderstanding this please let me know as I believe life is one long learning...

Glyn

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Hi Glyn,

The thermostat opens at 82 to allow water to the rads in order to attempt to regulate the water temp. It just remains open when the temp is at 82 Degrees and above feeding the rads with hot water in order to cool it. The effect of the differences in pressure cap occur above the temp of the thermostat.

Hi Artie

I agree that the water would boil at a higher temp and the damaging pockets of air caused during goiling would occur at a higher temperature.

If the thermostat opens at say 82 degrees, surely 82 degrees is 82 degrees? So how will the engine run cooler?

I would have thought that a higher pressure cap would allow for higher temps to be reached before the 'overheating' damage takes place, ie the gauge could show hotter before damage occurrs.

If I am misunderstanding this please let me know as I believe life is one long learning...

Glyn

Edited by CarlC

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